Remove 6.0 references from documentation

There are a number of 6.0 references throughout the source-build documentation that should be cleaned up once it reaches EOL.  

Examples:

- [Building .NET 6.0](https://github.com/dotnet/source-build/?tab=readme-ov-file#building-net-60)
- [Source build release template](https://github.com/dotnet/release/blob/main/.github/ISSUE_TEMPLATE/source-build-release.md)